Again, we look at particular topics in primatology because they have a special interest. This topic is interesting because the current theory on natural selection works on individuals, not groups, and we don't see too many behaviors that are good for someone else. This is one of these behaviors which looks like it's good for someone else, so it's very interesting to behavioral ecologists.
We can understand why non-reproductive males in polyandrous systems would help, as we talked about last time.
We can also understand why older siblings would help through the theory of inclusive fitness, because they are increasing their own reproductive success by helping out their siblings.
One thing they help out with is carrying. This is probably the biggest thing they do to help. They also sometimes provide food for offspring who aren't necessarily their own. Alloparents also guard them from predators or potentially hostile conspecifics. One form which is quite rare is nursing infants who aren't their own; even when a female is looking after an infant who isn't her own, she generally won't let it nurse. However, in cebus monkeys it has been observed on occasion.
The earliest observations of alloparenting behavior were back in the 40's in rhesus monkeys, in captivity. They documented allomothering by adult females who would carry around babies but weren't always nice to them.
Jane Lancaster showed that young nulliparous females were doing disproportionally much of the allomothering in vervets. In colobines like the leaf monkeys, sometimes an infant will be taken by another female right after it's born- when it has barely dried- and it will be passed from one female to another through the group, and the mom seems totally fine with it.
Moms obviously get a benefit but what are the costs? Babies don't always get treated great by the alloparent- sometimes there's a tug of war between two females to see who gets to hold the baby. Sometimes the alloparent doesn't carry it correctly- you see babies dragged around by their foot, sat upon, dropped. Also, sometimes the allomother will abandon the baby. These are risks to the baby and through it to the mom. So two questions arise;
We will tackle each question in term, but first some terminology;
parity= state of having had offspring before
nulliparous=not having borne offspring
primiparous=having borne one offspring
multiparous=having borne more than one offspring
Predictions Predicted by this
In langurs, it is true that nulliparous do show more interest in young infants than experienced females do. Sara Hrdy, who studied langurs for many years, wondered whether females were indeed more interested in allomothering before they'd have their own offspring. She found that nulliparous females tried to allomother more frequently than what you'd expected based on their proportion of the group's population, while parous females tried it much less than expected from their population in the group.
There is also data from vervets; based on their percentage of population, juveniles do disproportionally much of the allomothering compared to older females.
The other question is, does it actually help the allomothers become better mothers? Lynn Fairbanks studied vervets and found that first-time mothers with high alloparenting experience raised 100% of their first offspring to maturity, but mothers with low experience had less than a 50% survival rate of their first infant.
Is kin selection
You may be saying, 'Can we really dismiss kin selection so easily? It's so common; are we sure that's not what's responsible for the behavior of allomothers?' Basically, yes, they're sure. The main reason they don't think it's involved ;
"Kin selection is not implicated as a factor in the origin of allomaternal care.
Juvenile female vervets initiate caretaking about at same rates whether (3.5 bouts per hour) or not (3.6 bouts per hour) they had infant siblings in the group.
Female langurs manifest an interest in infants from other groups."
In fact, sometimes when females from two groups get together, females from one group kidnap a baby from the other group and hold it for sometimes hours despite the efforts of the mother to get it back.
Of course, kin selection is around; when given a choice, the number of caretaking bouts with siblings were much higher than expected given the number of nonsibs available compared to siblings available. This leads us to conclude that the kin directed part of this behavior is secondary to other motivations. Also, older kids don't always prefer their siblings.
Are the predictions supported by the
There is data from vervets, langurs, and squirrel monkeys that suggests that females feed faster and more efficiently when allomothering is available to them. All either feed longer or more efficiently when someone else is holding their infant.
The second prediction is also supported: When you compare the amount of time that a female's infant is carried by allomothers and how much time there is between her births, you see that as the amount of allomothering support goes up, the interbirth interval goes down. Thus, mothers who have babysitters can eat more, so they save up resources more quickly and can give birth again sooner.
Another idea which doesn't have as much data support: If another female has been allomothering for you and you're killed, then it's more likely that your infant will be adopted. Sort of like cultivating potential godmothers. Most evidence simply suggests that it frees mom up to forage more and reproduce faster.